By Sam Kim and Lee Haye-ah SEOUL, Nov. 22 (Yonhap) -- South Korea and the United States struggled Monday to restore confidence in their hard stance against North Korea after the communist country unveiled what may be its most advanced nuclear facility in a move seen as provocative.

Earlier this month, the North told a visiting Stanford University professor, Siegfried Hecker, that it had developed a uranium enrichment facility equipped with about 2,000 centrifuges.

The revelation, made public over the weekend, is one of the most sensational claims by the reclusive regime after a North Korean official admitted to a clandestine uranium program in 2002. This time, it came as Pyongyang bolstered its call for the resumption of six-party talks designed to denuclearize the country through aid.

Centrifuges, which Hecker said the North operated from an "ultra-modern control room," can be used to produce highly enriched uranium, an additional means of nuclear arms development for North Korea, which has already tested two plutonium-based bombs.

Uranium-based bombs pose a serious threat as their development process cannot be easily detected, while they are considered easier to transport, aggravating proliferation concerns.

North Korea told Hecker that it was enriching uranium to meet its peaceful energy needs, but senior U.S. officials, including Defense Secretary Robert Gates, have rejected the claim as false.

"This is obviously a disappointing announcement," Stephen Bosworth, the top U.S. point man on North Korea, told reporters Monday in Seoul, referring to the North Korean claim. "It is also another in a series of provocative moves."

"That being said, this is not a crisis. We're not surprised by this," Bosworth said, having met with South Korean Foreign Minister Kim Sung-hwan. "We have been watching and analyzing the (North's) aspirations to produce enriched uranium for some time."

Stephen Bosworth, U.S. point man on North Korea policy, holds a news conference on Nov. 22 in Seoul after talks with South Korean foreign ministry officials on North Korea's suspected uranium nuclear weapons program. (Yonhap)

Kim told Bosworth earlier that the allies "need more intelligence and analysis on what is the reality of the program" in North Korea, also downplaying the situation as "not a crisis."

Defending the U.S. goal of pressuring the North into taking dismantlement steps before reopening the six-party talks, stalled since late 2008, Bosworth said he "would not accept that our policy toward North Korea is a failure."

"We're not throwing our policy away," he said, pledging to "refine" approaches to North Korea because "we can't just ignore" the purported uranium program made public over the weekend.

"We have to incorporate this now into our strategy as we move forward," said the envoy, who arrived Sunday night here as part of a hurriedly arranged three-nation Asian swing.

Bosworth, set to travel on to Tokyo and Beijing, said it is "fundamental we deal with this in close coordination with major countries," and expressed hope for the eventual resumption of six-party talks.

"We have hope that we will be able to resuscitate it," he told reporters, referring to the negotiations that include the two Koreas, the U.S., Russia, Japan and China.

"My crystal ball is foggy, but I would never declare any process dead. It's still breathing," Bosworth said.

Japanese Chief Cabinet Secretary Yoshito Sengoku said in a press meeting in Tokyo that the North Korean claims of an operational uranium enrichment foundry are "absolutely unacceptable."

But a senior South Korean diplomat, speaking on the condition of anonymity, told reporters here separately that his government was considering the purported uranium enrichment activities as "real."

"It seems the situation must be accepted as is, and I believe it has become an issue we now have to deal with," he said, agreeing with Bosworth that the allies will not backtrack from their push for Pyongyang to show denuclearization steps before holding dialogue.

Bosworth made it clear earlier that his side wants the six-party talks to "take place under a proper set of conditions and in close coordination with our partners."

South Korean officials have previously called on the North to allow international nuclear monitors back on its soil and to move ahead with denuclearization steps agreed upon in a six-nation deal before the six-party talks can resume.

"It is fundamental that the North Koreans demonstrate that they approach the dialogue and the discussions in the negotiations with that same measure of seriousness and willingness to actually take hard decisions," Bosworth said.

The revelation of a uranium enrichment facility, located in Yongbyon north of Pyongyang, came after American security expert Jack Pritchard said the North has begun to build a light-water nuclear reactor, which uses low-enriched uranium as fuel.

A series of commercial satellite images released in the past week through the U.S.-based Institute for Science and International Security backs the claims of uranium activities in the North.

An image taken on Nov. 4 by DigitalGlobe shows that North Korea is laying the foundation for what it calls an experimental light-water reactor in Yongbyon. Another image unveiled on Monday showed a blue-roof building that is believed to be the one which Hecker said contained "hundreds and hundreds" of centrifuges.

The development of a highly sophisticated enrichment facility, if confirmed, could cast doubts on the policy of South Korea and the U.S., which sought to shut off any flow of supplies related to weapons of mass destruction into the North.

Bosworth and the South Korean official said their countries had no immediate plan to expand the raft of sanctions against North Korea, even though they denounced Pyongyang for violating a United Nations Security Council resolution against its nuclear testing.

"If North Korea has centrifuges as good as those in advanced countries, it will be able to produce 10 kilograms of highly enriched uranium every year, using 1,000 centrifuges," Lee Un-chul, a nuclear scientist at Seoul National University, said.

"With 2,000 centrifuges, one to two uranium bombs could be made in a year," he said, adding that he believes the North is capable of detonating uranium-based bombs.

Just three years after it conducted its first atomic test, North Korea detonated another device underground in May of last year. South Korean intelligence officials believe the North had also begun its hereditary power transition in earnest since at least early 2009.

It remains unclear how the succession process from leader Kim Jong-il to his third son Kim Jong-un, believed to be no older than 28, would bear on the expanding nuclear activities in North Korea.

The regime has ratcheted up its focus on the songun, or military-first, policy chartered by the senior Kim since the junior was unveiled to the world in September as a full military general.

"The nuclear program is tied to the succession plan involving Kim Jong-un, and its pace will be determined depending on whether it is favorable for the succession plan," said Baek Seung-joo, an expert at the state-run Korea Institute for Defense Analyses.

Japanese media reported last week that signs of preparations for another nuclear test were detected, but South Korean officials have dismissed them as lacking substance.

The series of developments on the peninsula comes as tension remains high between the two Koreas over the March sinking of a South Korean warship near their tense Yellow Sea border.

Forty-six sailors died in the sinking that Seoul blames on a North Korean torpedo. Pyongyang maintains its vehement denial of involvement, offering to give a torpedo part sample to challenge a multinational probe that found the North culpable in May.

In another development likely to exacerbate cross-border ties, South Korea's defense minister told lawmakers on Monday that his country was willing to consider allowing U.S. tactical nuclear weapons back on its soil to enhance deterrence against the North.

The U.S., which has 28,500 troops stationed here, declared in 1991 that it had withdrawn all of its tactical nuclear weapons from South Korea under its disarmament initiative. The defense ministry in Seoul later toned down the comment by Kim Tae-young, saying his remarks meant that "all possible options could be reviewed."